1. Field of the Invention
The present invention relates to thermoset polyurethane foam containing a brominated polymeric flame retardant, an isocyanate composition useful for making such foam and a method for making such foam.
2. Introduction
Thermoset polyurethane (PU) foam is a target for continuously changing and ever increasingly demands with respect to flame retardant and reduced smoke generation characteristics. Thermoset PU foam is commonly used in applications requiring the foam to meet specific flame retardant performance standards. Such applications include thermal insulation in building and construction. To achieve necessary flame retardant performance properties, thermoset PU foams often include brominated flame retardants. Flame retardants that are suitable for thermoset PU foam tend to be different from those that are suitable for thermoplastic polymer foam such as extruded polystyrene (XPS) and expanded polystyrene (EPS) foam. XPS and EPS foams soften and flow quickly away from the heat of a flame so flame retardants are designed to prevent ignition of a flame at the foam surface long enough for to the foam to move away from the flames. Unlike XPS and EPS foam, thermoset PU foam does not soften and flow from the heat of a fire because it is a heat stable thermoset.
Moreover, brominated flame retardant in XPS and EPS must be stable to release of bromine from the flame retardant molecule during processing temperature of at least 200 degrees Celsius (° C.) for extended periods of time. Such a stable flame retardant is not necessary, or even clearly desirable in thermoset PU foam. Thermoset PU foams generally do not exceed exothermic temperatures of 150° C. during manufacture, and if they do exceed 150° C. it is only for relatively short periods of time to avoid scorching the foam's interior. The higher stability of brominated flame retardants required for XPS and EPS is not necessary to survive the foaming process for making thermoset PU foam. Therefore, brominated flame retardants suitable for XPS and EPS foam are not necessarily suitable in thermoset PU foam.
Brominated flame retardants most commonly used in thermoset PU foam include aryl-brominated polyester polyols and aliphatic brominated small molecules such as dibromoneopentyl glycol, tribromoneopentyl alcohol and n-propyl bromide (n-PBr). Small molecule flame retardants, in general, have become undesirable because they have a higher tendency to migrate out from a material and enter the environment than larger molecules. In particular, n-PBr has a relatively high vapor pressure and tends to dissipate from thermoset PU foam over time. As a result, not only does n-PBr tend to migrate into the environment but flame retardant properties of thermoset PU foam containing n-PBr can gradually diminish over time as n-PBr is lost. Migration into the environment is particularly troublesome for small molecule brominated flame retardants, which tend to fall under regulatory scrutiny for being persistent and bioaccumulative. Therefore, small molecule brominated flame retardants are not desirable. There is a need for large molecule brominated flame retardants that are suitable for use in thermoset PU foam to avoid the problems associated with these small molecule brominated flame retardants.
Brominated polyols are somewhat larger molecules than n-PBr and are available for use in preparing thermoset PU foam as an alternative to n-PBr. Brominated polyols are reactive with isocyanate materials and as a result tend to consume isocyanate reactive sites and become bound into the backbone of PU polymer during polymerization. Brominated polyols are typically significantly more rigid than the non-brominated polyols and so reacting brominated polyols into thermoset PU foam tends to diminish resulting foam properties by, for example, increasing friability and dust formation associated with the thermoset PU foam. Another challenge with brominated polyols is their tendency to regenerate and migrate into the environment as the foam containing the brominated polyols undergoes environmentally induced decomposition through oxidation and/or hydrolysis. Therefore, it is desirable to find a non-reactive brominated flame retardant that has sufficient size so that it is unlikely to migrate from thermoset PU foam.
Published United States patent application US2011/0224320 and US2011/0224363 disclose brominated branched and star-branched styrene polymers that are taught as possible flame retardants in PU foam. The reference teaches dissolving the brominated polymer into a liquid phosphate flame retardant as a means for introducing both flame retardants into a PU foam formulation prior to reacting and foaming the formulation.
U.S. Pat. No. 7,585,443B2 discloses the use of brominated anionic styrenic polymer for use as a flame retardant for a variety of plastics including PU, though there is no mention of thermoset PU foam.
The brominated polymer flame retardants of US2011/0224320, US2011/0224363 and U.S. Pat. No. 7,585,443B2 contain bromine bound to aromatic rings. Bromine bound to aromatic rings (aromatic bromine) is particularly stable to bromine loss. As a result, the brominated polymer flame retardants of these references are require high temperatures or the use of small molecule synergists (such as polycumyl and/or antimony oxide) to release hydrogen bromide at the necessary temperature to inhibit combustion in thermoset PU foam. It is desirable to find an alternative brominated flame retardant that does not react with (consume) isocyanate sites, that has a lower stability than brominated aromatics, that does not require the presence of small molecule synergists and that are polymeric so as to have sufficient size to avoid migration into the environment.